# std::execution::read_env

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< cpp‎ | execution

Defined in header `<execution>` |  |
inline constexpr /*unspecified*/ read_env{}; |  | (since C++26) (customization point object)
Call signature |  |
execution::sender auto read_env( auto&& query ); |  | (since C++26)

A sender factory that returns a sender that reaches into a receiver’s environment and pulls out the current value associated with a given query object.

For any query object `q`, the expression read_env(q) is expression-equivalent to /*make-sender*/(read_env, q).

### Customization point objects

The name `execution::read_env` denotes a customization point object, which is a const function object of a literal `semiregular` class type. For exposition purposes, the cv-unqualified version of its type is denoted as `__read_env_fn`.

All instances of `__read_env_fn` are equal. The effects of invoking different instances of type `__read_env_fn` on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, `execution::read_env` can be copied freely and its copies can be used interchangeably.

Given a set of types `Args...`, if std::declval<Args>()... meet the requirements for arguments to `execution::read_env` above, `__read_env_fn` models

- std::invocable<__read_env_fn, Args...>,
- std::invocable<const __read_env_fn, Args...>,
- std::invocable<__read_env_fn&, Args...>, and
- std::invocable<const __read_env_fn&, Args...>.

Otherwise, no function call operator of `__read_env_fn` participates in overload resolution.

### Example

An example usage of this factory is to schedule dependent work on the receiver's scheduler, which can be obtained with read_env(get_scheduler):

    std::execution::sender auto task =
      std::execution::read_env(std::execution::get_scheduler)
        | std::execution::let_value([](auto sched) {
            return std::execution::starts_on(sched, /*some nested work here*/);
        });

    std::this_thread::sync_wait( std::move(task) ); // wait for it to finish

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